The present invention relates to screen printing apparatus in general, and more particularly to improvements in screen printing apparatus wherein the screen is a rotary body, such as a cylinder.
It is already known to provide a screen printing apparatus with a cylindrical screen whose interior receives the liquid medium (hereinafter called ink) to be applied to a moving stock, such as a thin web of textile material, a carpet or the like. The ink accumulates in the lower portion of the interior of the rotating screen and forms therein a pool whose contents drip through the pores of the screen to descend onto the stock therebelow. The screen need not be a cylinder; it can also constitute a band, a stencil, a stencil band or the like. Such apparatus are used in screen printing machines for a variety of purposes, e.g., to impart to the stock a pattern consisting of a single color or of several different colors, or to simply color the stock if the pores of the screen do not represent an image.
In accordance with a normal screen printing technique, ink which is to be applied to the stock is spread over the stock by resorting to squeegees in the form of rollers, slotted or unslotted doctor blades or the like. Such squeegees cause ink to penetrate through the pores of the screen and to form an image on the stock therebelow. The application of patterns to webs of fabric or the like is desirable and advantageous in many instances. However, if the apparatus is to cover large or very large areas, e.g., in connection with the treatment of various forms of carpeting, it is not always desirable or advantageous to repeat the same pattern all over again because it is difficult to accurately match repetitive patterns in the regions of the so-called seams. Accurate matching is particularly difficult if the area to be covered is large or extremely large. Therefore, the trend in the industry is to effect a so-called dispersion of the patterns.
It is further known to resort to cylindrical screens in connection with the so-called uni-printing. To this end, the pores are distributed uniformly in the entire screen to thus ensure that the application of ink to the stock will be uniform. This is often desirable or necessary when the applied medium is a chemical substance. As a rule, the just discussed technique should result in uniform coloring or imprinting of the entire stock which can thereafter be provided with one or more patterns in the course of a secondary treatment, e.g., by different structuring of the substrate, by resort to various light and shadow effects and/or others.
German Auslegeschrift No. 19 13 175 discloses the possibility of installing the screen at a certain distance from the stock. The interior of the cylindrical screen contains a pressure generating and backup squeegee which is immediately adjacent to a counterpressure generating device constituting a second squeegee. The screen is installed at a level above the path of the stock and the apparatus operates in such a way that the squeegees together constitute a weir the upper part of which is located in the interior of the cylindrical screen and the lower part of which is located at a level below the screen. Thus, the weir serves to accumulate ink first in the interior of the screen, and such ink descends by gravity and flows along the lower part of the weir, namely, along the second squeegee, to form a more or less uniform stream extending along the full length of the cylindrical screen. A drawback of such apparatus is that the ink cannot descend by gravity directly onto the stock; instead, the descending ink is converted into a shallow stream on its way from the interior of the screen toward the path of the stock. Therefore, if the pores of the screen form one or more patterns, such pattern or patterns disappear when the ink issuing from the pores is converted into a stream before it reaches the stock. In other words, the entire pattern disappears and the stock is merely coated with a more or less uniform layer of ink. Such equalization of the outflowing mass of ink is evidently undesirable if the pores of the screen form an image or pattern since the utilization of a patterned screen or stencil then serves no useful purpose. Thus, the situation is the same as if a screen whose pores form one or more patterns were replaced with a screen which is formed with uniformly distributed pores.
Another drawback of such apparatus is that the first squeegee removes ink from the screen so that each increment of the screen is clean when it begins a fresh revolution. This is not conducive to the formation of desirable patterns on the stock.